Equilibrium and Transport of Radiation
The quantized vibrations of the electromagnetic field are equivalent to a gas of bosons, the photons, non-interacting relativistic particles with zero rest mass, which can be created or annihilated by matter (§ 13.1.4). The thermal equilibrium of the radiation in an enclosure is characterized by the Planck distribution (13.42) of the energy as function of the frequency. The total energy and the pressure of the radiation vary as T4.
A nearly closed enclosure emits as a perfectly absorbing black body. Black bodies are those which radiate the most strongly, according to (13.44) and (13.45). Kirchhoiff’s law (13.54) which connects the luminance with the absorptivity helps to determine the radiation from other bodies, and has many practical applications.
KeywordsElectromagnetic Field Thermal Equilibrium Maxwell Equation Cosmic Background Radiation Black Body
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